Waveform signals often propagate through circuits, such as integrated circuits. Often these signals are periodic and designed to be identical. Propagation delays, circuit elements, and the like may cause such waveform signals to shift relative to one another. Time-to-Digital Converters (TDC) measure the relative shift between two otherwise identical waveform signals. These devices typically operate at high power. Though these devices are highly calibrated to address noise and offset issues, such converters are limited in the resolution at which they can measure such phase differences between signal waveforms.
Take, for example, two square wave clock signals having the same frequency with repeating rising and falling edges. The rising edge of a first of the clock signals may occur prior to a corresponding rising edge in the second of the clock signals. The TDC measures that amount of time between a rising edge (for example) of the first of the clock signals and a corresponding rising edge of the second of the clock signals. The TDC converts this duration into a digital value, but the smallest resolution the device can provide may be too limited based on the needs of the user. Also, noise and other factors may lead to errors in the measurement of phase difference in the TDC.
Accordingly, there is a need for an improved method and apparatus for measuring phase difference between signals.